Field of the Invention
The present invention, in the preferred form, relates in general to workpiece gripping and manipulating apparatus for use with article handling devices for quick connection and disconnection with a workpiece and transfer of the workpiece to and from a processing station for manipulation through a sequence of predetermined positions, and more particularly to workpiece gripping and manipulating apparatus for use in a computer controlled precision laser welding system for quick connect gripping and release and positioning of fixtured workpieces, specifically fuel rod grids for nuclear fuel assemblies.
The workpiece gripping and manipulating apparatus of the present invention is specifically designed for use with precision laser welding apparatus designed to manufacture nuclear fuel bundle assemblies by computer controlled laser welding apparatus as disclosed in detail in the related companion patent application designated Ser. No. 414,241 in the names of Don L. Wolfe, Jack W. Clements and J. S. Kerrey. As described more particularly in that related patent application, the laser welding system includes a pair of positioning modules and associated welding chambers, wherein fixtured workpieces formed of an array of metal straps arranged in proper position to be laser welded at various intersections and slot and tab locations are positioned on a mounting fixture and are to be transferred into the welding area for aligning and positioning in a computer controlled series of steps. As described in the related patent application Ser. No. 414,241, the prior art has recognized the problem of fretting corrosion wherein the surfaces of fuel rod grids and fuel rods in nuclear fuel assemblies rub against each other increasing the likelihood of weld contamination and eventual mechanical failure of the fuel rod grids. Fuel rod assemblies including the fuel rods and grids are designed to be disposed within the hostile atmosphere of a boiling water reactor (BWR) or pressurized water reactor (PWR), wherein the coolant typically in the form of water is superheated to temperatures of the order of 600.degree. F. Under such conditions, any contamination and in particular fretting corrosion is enhanced. It has been recognized in the literature that the welding of grid and rod materials such as zirconium alloys in a contaminated welding atmosphere leads to contaminated welds which may bring about eventual joint failure in the nuclear fuel assembly. In particular, zirconium alloys are especially prone to direct oxidation of the metal by scraping action encountered in the nuclear reactors. Experience has shown that welds produced in a relatively impure atmosphere will result in a weld with an initially low degree of contamination that, when subjected to the harsh atmosphere of a nuclear reactor, will be particularly subject to fretting contamination. Thus, it is particularly critical that any welding of Zircaloy and in particular, laser welding, be conducted in a controlled, pure atmosphere to insure that weld contamination is minimized and will not deteriorate under the hostile conditions of a nuclear reactor.
The companion related U.S. patent application, Ser. No. 414291, thus discloses a system for achieving automated laser welding of highly reactive materials such as Zircaloy, wherein the workpiece is sequentially moved under an automated controller to effect a number of precision welds, while maintaining an exceptionally high degree of purity of the surrounding atmosphere to avoid contamination of the welded material. To achieve the precise welds required with parts of small dimension, making up the nuclear rod grids, the Zircaloy straps precisely positioned as required in the finished grid are positioned in a fixtured grid assembly, which must be introduced into and withdrawn from associated welding chambers in positioning modules in as efficient a manner as possible and in a way which achieves the goal of minimum contamination.